This invention relates to the stabilization of methylene chloride, and more particularly, it relates to the stabilization of methylene chloride against degradation when exposed to heat, light and air.
Methylene chloride (CH.sub.2 Cl.sub.2, boiling point 40.1.degree. C.) is a highly versatile and useful solvent for various industrial applications at both normal and elevated temperatures. It is the least toxic of the chloromethanes. Its outstanding solvent properties are the principal basis of its industrial interest. A particularly important industrial use of methylene chloride is in the vapor degreasing of metals.
Among the properties which make methylene chloride so attractive in degreasing applications is its greater stability than other chlorinated hydrocarbon solvents such as perchloroethylene, trichloroethylene and methyl chloroform. For example, methylene chloride is more resistant to oxidation, hydrolysis and pyrolysis than other chlorinated solvents. Additionally, methylene chloride can be used to greater advantage than other degreasing solvents in vapor degreasing metals, since it can be used effectively at lower temperatures due to its lower boiling point and excellent stability. Methylene chloride is particularly desirable for degreasing since it is substantially resistant to photochemical activity and, therefore, does not contribute to air pollution by smog formation.
Methylene chloride does have disadvantages when used in certain metal cleaning functions, including vapor degreasing. For example, methylene chloride can react with aromatic and aliphatic compounds in the presence of metals, metal halides and combinations thereof, including aluminum, zinc and iron, halides thereof, and combinations of said metals and halides. The reaction product is generally an objectionable high boiling tarry substance which renders the methylene chloride unsuitable for further use.
Compounds that react with methylene chloride to produce tarry substances are generally introduced into the methylene chloride from various cutting oils and lubricants used in metal fabricating operations, which are carried over into the methylene chloride solvent during vapor degreasing or other cleaning of the fabricated metal parts. Solvent manufacturing, handling and storage equipment are other sources which can introduce these impurities.
Minor quantities of various organic compounds have been incorporated into methylene chloride to prevent degradation and other types of deterioration such as oxidation, hydrolysis and pyrolysis. These organic compounds act as stabilizers to substantially prevent degradation of methylene chloride and to inhibit reactions which can lead to decomposition and corrosion.
In recent years, neutral stabilizing systems for methylene chloride have been found to be particularly advantageous. These systems generally contain as a principal component, an essentially neutral compound which acts as an acceptor of strong acids such as hydrochloric acid, but does not ordinarily react with weak organic acids. This neutral compound is generally an epoxide such as butylene oxide, propylene oxide, epichlorohydrin, butadiene dioxide, styrene oxide, glycidol, pentene oxide, cyclohexene oxide, or a mixture of two or more of these.
Many additives have been suggested as stabilizers for methylene chloride, nevertheless, there is still a great need for improving the stability of CH.sub.2 Cl.sub.2 at high temperatures. When methylene chloride is stabilized, its tendency towards reaction is diminished. However, during the course of high temperature applications, stabilizers have a tendency to degrade, thereby increasing the tendency of methylene chloride to react. This situation has made the stabilization of methylene chloride over long periods of time, e.g. two to three weeks, at high temperature very difficult.
It has, therefore, been an arduous and empirical task for practitioners in the art to develop a suitable stabilizing formulation for methylene chloride which extends the period of stabilization at high temperatures. The present invention has achieved an efficient and commercially economical stabilizer package for methylene chloride which enables the use of methylene chloride at high temperatures over long periods of time.